Polypropylene compositions have traditionally been used to make film products. However, problems have been associated with the processability of polypropylene compositions and the film products containing such polypropylene compositions. For example, in the production of cast films, extruded polypropylene melts often stick to the chill roll, which results in reduced production yield and efficiency. Polypropylene film products, particularly those from polypropylene homopolymers, are relatively brittle and have low impact resistance. These film characteristics create disadvantages of unpleasant texture, excessive noise and susceptibility to tearing.
Numerous procedures have been proposed for modifying the properties of the polypropylene homopolymers to improve their characteristics. Many of those proposals have involved incorporating a propylene/.alpha.-olefin copolymer portion in an otherwise homopolymeric polypropylene. The structure of such modified polypropylene polymers is variable, but somewhat uncertain.
For example, Leibson et al., U.S. Pat. No. 3,514,501, describes a process for the production of block copolymers wherein a homopolymeric polypropylene prepolymer is produced and a block of at least one .alpha.-olefin is grown from the prepolymer in a second polymerization step. Another approach involves the mixing of a polypropylene homopolymer with a propylene/ethylene copolymer. Most commercial products of high impact strength result from the production of a first polymer, usually a polypropylene homopolymer, and the production of a copolymeric portion in the presence of the initial polymer product mixture which still contains active polymerization sites. Whether the resulting product is a true block copolymer, a mixture of homopolymer and copolymer or is of another structure is not entirely clear. They are often referred to as "polypropylene impact copolymers," regardless of the precise nature of their structure. Polypropylene impact copolymers are said to contain a homopolymer phase and a copolymer phase. The homopolymer phase is often homopolymeric polypropylene and the copolymer phase is often a rubber composition.
McCullough, Jr. et al., U.S. Pat. No. 5,362,782, describes stress whitening resistant polypropylene impact copolymer compositions which contain a homopolymer phase of at least 94% polypropylene, a copolymer phase of propylene and ethylene, and a nucleating agent, wherein the ratio of the intrinsic viscosity of the copolymer rubber phase to the intrinsic viscosity of the homopolymer phase is from about 1.1 to about 1.3. McCullough, Jr. et al., U.S. Pat. No. 5,250,631, describes more stress whitening resistant polypropylene impact copolymer compositions which contain a homopolymeric phase of predominantly homopolymeric polypropylene and a terpolymer phase having a major proportion of ethylene, a minor proportion of an .alpha.-olefin of at least 4 carbon atoms and a minor proportion of propylene. Neither of these references refers to polypropylene compositions to be used in the production of soft, quiet films.
Other references including Sumitomo, U.S. Pat. No. 3,517,086; Sumitomo, U.S. Pat. No. 4,902,738; and El Paso, U.S. Pat. No. 4,704,421, discuss polymeric compositions which may be used to produce films that are "soft." However, none of these references refer to a polypropylene impact copolymer containing a nucleating agent and having a rubber content (Fc) of greater than 25% by weight.
A need exists in the art of polymer compositions and cast films for polypropylene polymer compositions having good processability and characteristics sufficient to produce soft, quiet cast films.